HR 8799 is a variable star located 135 light-years from the Sun, in the constellation Pegasus. It is a young star, around 30 million years old, with 1.5 times the mass and 4.9 times the luminosity of the Sun. Around it orbits a planetary system comprising a debris disk and at least four massive exoplanets.
In 2008, this system was one of the first to be observed by direct imaging, using the Keck telescope equipped with adaptive optics.
Thanks to the coronagraph of the MIRI instrument, the James Webb Space Observatory is perfectly equipped to probe such a multi-planetary system, enabling it to be explored in the mid-infrared, a wavelength range never before used.
| ◼ The coronagraph of the James Webb Telescope The Laboratoire d’études spatiales et d’instrumentation en astrophysique de l’Observatoire de Paris - PSL, in collaboration with the CEA’s AIM laboratory, has been commissioned to supply the coronagraphs for the MIRI instrument for the Webb Telescope ; these are of a new type, based on a technology developed at Observatoire de Paris - PSL in the 2000s : the four-quadrant phase mask. Three of these masks, plus a so-called "Lyot" mask (named after the inventor of the solar coronagraph in the 1930s, at Observatoire de Paris - PSL) are installed in the MIRI instrument, whose imaging system was designed and built in France. This system now offers high-contrast imaging capability, making it possible for the first time to obtain images of exoplanets in a previously unexplored wavelength regime : from 10 to 20 microns. |
HR 8799 was the first coronagraphically observed target in the MIRI-EC Guaranteed Time Observations (GTO) exoplanet program, launched in November 2022.
First target observed by MIRI’s coronagraph
Observations carried out in November 2023, with very high contrast imaging capability, made it possible to attenuate the star’s luminous flux to make its surroundings visible.
Thanks to cutting-edge technology in mid-infrared photometric measurements, combined with near-infrared observations from the ground, a new, finer estimate of the diameter of these planets has been made possible : between 1 and 1.5 times the radius of Jupiter. This updated figure seems more consistent with the predictions of planetary evolution models, given their age of 30 million years.
What’s more, while only three of the four known planets were expected to be detected, the sensitivity of MIRI’s coronagraph enabled it to capture the fourth as well, a feat given that it is the closest to the star HR8799.
The presence of ammonia in the atmosphere, which was sought in the coolest of the four planets, could not be confirmed.
Finally, the internal disk, made up of micrometric dust, had never been imaged before. This has now been done. Its presence implies that there are probably no other giant planets within 15 astronomical units of the star.
Other young exoplanetary systems will be observed with MIRI’s coronagraph, with particular emphasis on the search for an ammonia signature for planets with temperatures below 1,000 Kelvin, with the aim of increasing our knowledge of exoplanet formation mechanisms.
| On vidéo |
Reference
Imaging detection of the inner dust belt and the four exoplanets in the HR 8799 system with JWST’s MIRI coronagraph, by Anthony Boccaletti et al. Astronomy & Astrophysics (A&A), 686, A33 (2024)